1. Field of the Invention
The invention generally relates to electronics. In particular, the invention relates to filtering of interference.
2. Description of the Related Art
Co-existence of wireless communication links from different wireless standards, and a generally crowded wireless spectrum results in “interfering” radio signals near the frequency of a desired radio signal to be received, as illustrated in FIG. 1.
In an extreme case, the presence of a relatively large interferer near the desired signal makes reception of the desired signal impossible. Even in a relatively good case, the ability to handle a relatively large interferer increases the linearity and baseband filtering requirements of the radio, which in turn increases the radio's cost and power.
One conventional solution to the problem of a large interferer is to increase the linearity and increase the analog baseband requirements of the radio front end. This approach increases both the cost and the power used by the radio.
In another approach illustrated in FIG. 2, the interfering signal is separated at baseband, then up-converted to RF and subtracted from the total RF signal. See Aminghasem Safarian, et al., Integrated Blocker Filtering RF Front Ends, Radio Frequency Integrated Circuits (RFIC) Symposium, Jun. 3-5, 2007, 2007 Institute of Electrical and Electronics Engineers (IEEE), pp. 13-16. Also see A. Mirzaei and H. Darabi, A Low-Power WCDMA Transmitter With an Integrated Notch Filter, IEEE Journal of Solid-State Circuits, Institute of Electrical and Electronics Engineers (IEEE), Vol. 43, No. 12, December 2008.
Receivers for wireless radio typically tolerate interfering signals (interferers) in two basic duplex scenarios: Time Division Duplex (TDD) and Frequency Division Duplex (FDD).
FIG. 9 illustrates an example of a spectrum of channels that can be observed in Time Division Duplex (TDD) systems. In a TDD system, a receiver and a transmitter of a transceiver operate at different times. However, co-located wireless transceivers that operate near the same band as a desired signal, such as in the case of Wi-Fi and WiMAX transmission, can result in relatively large nearby interferers.
In the illustrated example of FIG. 9, a WiMAX transceiver may be receiving a signal centered at 2,501 megahertz (MHz), while a nearby Wi-Fi transceiver may be transmitting a signal around 2,472 MHz.
In a Frequency Division Duplex (FDD) system, a transceiver's own transmitter can be transmitting at the same time that it is receiving a signal. Due to the finite amount of attenuation of the local transmitter signal into the receive signal by the duplexer and other filtering, there may exist a residual transmit signal received with the received signal sufficient to cause distortion.